1. Field of the Invention
This invention relates to an optical device of a single lens reflex camera which is capable of focus detection, and more particularly to an optical device which directs two light beams passed through different areas in the pupil of the phototaking lens of a single lens reflex camera to photoelectric conversion elements to thereby effect focus detection.
2. Description of the Prior Art
As an optical device for focus detection of a single lens reflex camera, there is known one in which two light beams leaving a point Q on an object and passing through two different areas in the pupil of a phototaking lens are imaged on a finder screen via a mirror for finder, and further, from a light path passing through the screen and a pentaroof prism to a finder eyepiece, the two light beams split by a beam splitter are re-imaged on separate photoelectric conversion elements. Such optical device includes a detecting optical system for accurately directing the two light beams split by the beam splitter to the separate photoelectric conversion elements. One of the most important problems in such a detecting optical system is how the detecting optical system can be compactly and accurately incorporated into the perimeter of a finder optical system including a pentaprism and an eyepiece. This is because, in the principle of this system, the deviation of the point Q on the object or the deviation of the focus of the phototaking lens on the focal plane of the screen resulting from the movement of the phototaking lens along the optical axis thereof is detected by the above-mentioned two light beams being deviated to left and right on the focal plane and these changing places with each other on the left and right with the position of the focus as the boundary. Accordingly, each element of the optical system for accurately separating and detecting the two light beams must be constructed sturdily and readily machinably so that geometrical-optical, physico-optical and photoelectrical conditions including the configuration, position and characteristic of the photoelectric conversion elements are equal and adjustment is easy to accomplish and difficult to break down. However, the techniques heretofore proposed cannot always be said to satisfy these conditions.
For example, the camera disclosed in Japanese Laid-open Utility Model application No. 37534/1978 has, on the opposite sides of a finder eyepiece provided on the exit surface of the pentaprism, a detecting optical system having a pair of detecting lenses disposed for re-imaging light beams on photoelectric conversion elements. However, if the detecting lenses are disposed in this manner, the optical distance between the finder screen and the detecting lenses will necessarily become great. Making it a proviso that the effective light-receiving surface of the photoelectric conversion elements and the range to be measured of the object being imaged on the finder screen are conjugate with the detecting lenses, an increase in the distance between the finder screen and the detecting lenses brings about a reduction in the light-receiving surface and further, the entire device cannot be compactly constructed unless the photoelectric conversion elements are brought extremely close to the detecting lenses and therefore, the effective light-receiving surface becomes very small. Accordingly, the accuracy of proper disposition of the effective light-receiving surface becomes very severe. Similar disadvantages will also be encountered where the beam splitter is provided within or closely adjacent to the eyepiece.
Conversely, where the detecting lenses are disposed as close as possible to the finder screen, the two light beams exiting from the screen are not yet sufficiently separate from each other at this position and, in a detecting optical system having a sufficiently large opening, this leads to a disadvantage that it is difficult to accurately separate and take out the light beams passed through two areas in the pupil of the phototaking lens and imaged on the screen.
When these points are taken into account, it is desirable to select a value of projection magnification approximate to 1 with respect to the effective light-receiving surface on the photoelectric conversion elements which corresponds to the area to be distance-measured on the screen.
Among the conventional techniques, there are ones in which two light beams immediately after having left the beam splitter are bent upwardly and these light beams are caused to intersect the finder eyepiece optical axis between the eyepiece and the pentaprism or are directed from below to above in the eyepiece (Japanese Laid-open patent application No. 121122/1979), but neither of these techniques are free of the disadvantage that the inverse incident light from the finder eyepiece optical system reaches the photoelectric conversion elements as a flare light to reduce the S/N ratio.
On the other hand, there are conventional single lens reflex cameras in which a light beam split by a beam splitter disposed in a finder optical system is directed onto a photoelectric conversion element through a focus detecting optical system including a filter for cutting lights outside the visible range (for example, infrared light). This filter is for preventing the photoelectric conversion element from reacting to lights outside the visible range and is provided in the focus detecting optical system and can theoretically cut, in addition to lights outside the visible range which are directed to the photoelectric conversion element through the phototaking lens, quick return mirror, beam splitter and focus detecting optical system, the so-called inverse incident light outside the visible range which is directed from the eyepiece portion to the photoelectric conversion element through the eyepiece, pentaprism, beam splitter and focus detecting optical system, but actually does not perform a sufficient function to eliminate the influence of the inverse incident light, particularly the influence of lights outside the visible range.